This invention relates to machinery for mixing, working, homogenizing and/or compounding a flowable medium by itself or with others and/or with solids, liquids or gases. It is particularly though not exclusively concerned with plastifyable visco-elastic materials such as rubber and meltable materials such as plastics.
Continuously operating mixing apparatus is known with what has become accepted in the art as a Transfermix geometry, generically defined in B.P. No. 842'692 as:
Apparatus comprising a component having an internal operating surface provided wish a helical thread and an interior component having an external operating surface provided with a helical thread of a different direction but coaxial with the said helical thread on said component having said internal operating surface, the said helical threads facing one another and defining a passage for a medium to be mixed; the envelope defined by the crown of the said interior thread being within the envelope defined by the crown of the said exterior thread up to and including coincidence of the said envelopes, and the cross-sectional areas of the grooves of the said facing threads varying in opposite senses between a maximum and a minimum value for of each of the said threads along substantially the same length of the said passage; whereby when in operation the said medium is caused to move in the said passage, portions thereof are transferred successively between the grooves of the said facing helical threads as giver and taker.
Such apparatus may be stationary and the medium may be pumped.
In such apparatus, the minimum values of groove cross sectional area may be zero, and the said threads may be of opposite hand.
Continuously operating extruders are known in which a rotor and a barrel form an inlet-section, a Transfermix section and an exit-section, the said rotor having said external helical thread and the barrel having in the said Transfermix section the said internal helical thread of opposite hand but coaxial with the helical thread on the rotor, the thread in the one component e.g. the rotor varying from a full-cross-section to zero-cross-section over the axial length of the Transfermix section while the thread in the other component (e.g. the barrel) varies from zero-cross-section to full-cross-section, and vice versa in a possibly following Transfermix section.
In operation, the material being transported in the one component at the entry of this Transfermix section is transferred layer-by-layer from this as the giver-component to the taker component until by the end of the mixing section, all the material will have been transferred, mixed and worked transversely, layer by layer in orderly sequence.
Two generations of such Transfermix are described in GB Patents Nos. 842'692; (Frankel) and Nos. 1'585'531 and 1'585'532 (Meyer).
Such Transfermix have been applied to the mixing and extrusion of rubber compounds, plastics and of various viscous substances in Chemical Engineering in each case providing superior performance in transverse or cross-sectional mixing. This has meant that care had to be taken that all she ingredients to be mixed were present in the required proportion in each cross-section of the annular flow that was presented at the entry to the Transfermix section, if uniformity to a required degree was to be achieved at the exit from such a Transfermix section.
This has necessitated preparation in a preceding blending operation to the standard of equal proportioning required, whereas frequently a degree of longitudinal mixing incorporated with the cross-wise mixing would do away with some or all of such preparation.
An example of such a requirement for rubber mixing is in a Transfermix as a dump-extruder following an Internal Batch Mixer of a Banbury or equivalent type, as are generally installed in the Rubber Industry as primary mixers. The mixing cycle-time in such an Internal Mixer could generally be reduced considerably if a Transfermix hot-fed dump extruder had longitudinal as well as cross-wise mixing capacity.
This would make possible the upgrading of existing Internal Mixer lines which generally at this time have either mill-trains following them, for further mixing without additional heating-up, or simple, non-mixing dump-extruders for shaping the mix into sheet or pellets for subsequent cooling. To achieve the necessary mixing for many compounds, two or more mixing cycles through the installation are needed.
There are also requirements for longitudinal mixing in cold-fed Transfermix, in order to simplify a preparation of the feed or to eliminate such preparation altogether.
Transfermix for the Rubber and Plastics Industry have generally been conceived as extruders with a driven rotor and a stationary barrel and have not exceeded diameters of 24/21" or 610/530 mm with rotational speeds of 3-12 r.p.m., necessitating very large and costly gear-trains.
However, for the Earth, Chemical and Metallurgical Industries having to deal with large lumps of earth or filtercake, Transfermix have been conceived with diameters of a bigger order of magnitude, say 1000 to 3000 mm upwards. In these, the internal component would be stationary and an outer one (barrel) would be driven in the manner of a rotary kiln at low speed, and having either a horizontal or a vertical axis.
The drives of such devices would be simpler than the gear-boxes usual for extruders in that a gear-ring of large diameter fitted around the outside of the barrel would be driven by a standard gear motor. Any bigger powers required could easily be accomodated by arranging more such standard gear motors around the circumference.
Hot-fed Transfermix having also longitudinal mixing capacity conceived with rotated barrels of larger diameters and simplified drives as for kilns could be utilized as dump-extruders after Internal Mixers in the Rubber and Plastics industry.
A very crude comparison of the cooling surface available on even the biggest mills (84" length.times.26".phi. or 2,100 mm length.times.665 mm ), taking twice the area of the banded roll as the effective cooling area (the outside radiating off heat) will show that, for example, a 1,500 mm mean diameter Transfermix of 1,500 mm effective length will have more cooling surface than two 84" mills in sequence. Such a Transfermix of 3,000 mm length will be able to exert correspondingly more cooling.
This opens up the prospect of using dump-Transfermix under Internal Mixers which like or better than mill-trains apply cooling as well as mixing, where it is known that the lower the temperature of the mix the better will be the quality of the mixing in respect of incorporation of fillers such as carbon-black.